A lowpass filter whose cutoff frequency is controlled by a voltage or current is useful for many signal processing applications. When the responance of the filter is similarly controllable, a completely electronically programmable lowpass filter results. This is particularly useful for computer controlled signal processing where a computer generates, via digital-to-analog converters, voltages to control the filters.
A voltage or current controlled filter for audio use should have a good signal/noise ratio and low distortion. Additionally, the filter should reject the control signal to a high degree. That is, the output of the filter should be purely the lowpass function of the input signal, and the filter should reject components of the cutoff control voltage signal. An optimum circuit should be inexpensive to produce, and would be particularly attractive if the circuit were applicable to integrated circuit technology. Finally, the resonance control of the filter should be independent of the cutoff frequency, and should provide for a stable resonance value as the cutoff frequency is swept over a wide range.
Prior art voltage and current controlled lowpass filters present several techniques, each with its own shortcomings. Problems such as poor control signal rejection, poor signal to noise ratio, requirements for AC coupling or level shifting and high component count are typical prior art disadvantages.